This invention relates to a method of utilization of tracer gases for localizing failures which are liable to occur in the cans of nuclear fuel pins.
The invention further relates to a novel detecting capsule which is mounted within the fuel pins and to pins fitted with capsules of this type.
It is known that any failure of a certain number of fuel cans results in the release of radioactive products into the reactor, causes considerable disturbances in the operation of the reactor and even leads to shutdown.
There is one known method of localization which consists in labelling all the fuel pins of an assembly by means of a specific tracer gas. In the event of can failure, it is only necessary to determine the composition of the released tracer in order to detect the particular fuel assembly in which the can failure has taken place. Accuracy of localization is greater as the number of available tracers is larger.
In the case of a fast reactor, this known method entails the use of mixtures of natural xenon and of xenon isotopes. If it is desired to label all the fuel assemblies in the reactor, it may be found necessary to provide a large number of tracers and consequently to employ very high isotopic enrichments, thus entailing extremely high capital costs. It is consequently an advantage in this case to employ stable tracer gases constituted by isotopic mixtures of xenon containing at least one of the following isotopes: EQU Xe.sup.124, Xe.sup.126, Xe.sup.128, Xe.sup.129
And/or of krypton containing at least one of the following isotopes: EQU Kr.sup.78, Kr.sup.80, Kr.sup.82
These particular isotopes permit a higher degree of selectivity since they are not found, or are found only exceptionally in the state of traces, either in the fission products or in the state of impurity.
The mixtures of isotopes of xenon and of krypton can be introduced alone but can also be introduced in a mixture, thus making it possible to increase the number of combinations which may be contemplated and to extend the range of detection potentialities.
In regard to the method of labelling of fuel pins by means of tracer gases, the solution which immediately comes to mind consists in introducing the selected gaseous compound into the interior of each fuel pin. In practice, the application of this solution proves to be both difficult and costly since it complicates the production cycle to an appreciable extent. Thus it is necessary to work in a glove box atmosphere or to have recourse to the use of plugs with tubular stems involving the successive steps of filling and then sealing.
Furthermore, such methods do not permit the possibility of checking the quantity of tracer gases introduced into each fuel pin, thereby entailing reduced sensitivity in the localization of fuel can failures. It may even happen that some fuel pins do not contain radioactive isotopes or in other words are not labelled since there is no possible means of detecting this deficiency prior to putting the pins into service.
This invention relates to a method which makes it possible to introduce the same quantity of tracer gases into each fuel pin while being simple to apply since it can be carried out in conventional workshops.